River network position mediates dispersal connectivity and environmental filtering in shaping benthic communities

Understanding how river network position (RNP) shapes biodiversity patterns is critical for unraveling metacommunity assembly mechanisms in riverine ecosystems. However, the influence of RNP on the replacement and nestedness components of beta diversity remains underexplored. Here, we examined benthic macroinvertebrates and diatoms across six tributaries of the upper Jinsha River on the Tibetan Plateau, China. RNP was quantified using river order, upstream drainage area and three graph-theoretical metrics: betweenness, closeness and Harary centrality. Beta diversity was assessed using Local Contributions to Beta Diversity (LCBD), with three indices calculated: LCBDTotal (overall compositional uniqueness), LCBDRep (species replacement) and LCBDNes (species nestedness). Our results revealed that LCBDTotal and LCBDRep of benthic macroinvertebrates and diatoms decreased as betweenness centrality increased, while LCBDNes remained unaffected. Moreover, environmental variables including water depth and flow velocity also significantly influenced LCBDTotal and LCBDRep. Turbidity significantly affected LCBDTotal of both macroinvertebrates and diatoms, whereas total nitrogen affected only LCBDTotal of diatoms. We further found that the joint effects of betweenness centrality and important environmental variables on LCBDTotal and LCBDRep exceeded their independent effects, indicating that RNP captures not only spatial configuration but also underlying environmental variation along the river network. Additionally, river sites with low betweenness centrality, < 800 for diatoms and < 1710 for benthic macroinvertebrates, emerged as critical nodes structuring community composition. Our findings highlight that RNP simultaneously governs metacommunity assembly through dispersal connectivity and habitat filtering, thereby underscoring the need to integrate network topology into strategies for conserving biodiversity in dendritic river systems.